Silicon photonics has emerged as a mature technology platform for applications in optical data communications and telecommunications. High quality photonic integrated circuits (PICs) can today be fabricated with good process control and yield. State-of-the-art devices such as optical waveguides are frequently used on this platform. One useful optical component on these circuits is the nanophotonic waveguide which is used to facilitate optical communication between different optical devices on the circuit.
Nanophotonic waveguides have relatively small physical dimensions. One challenge is therefore to achieve an efficient light coupling into, and out from, the waveguide. One method for coupling light into a waveguide is in-line coupling where the light is coupled into the waveguide directly at one of its ends. The small cross sectional area of the waveguide requires accurate alignment to obtain high coupling efficiency. The coupling efficiency will be strongly dependent on the relative alignment of the components, making in-line coupling unsuitable for many practical applications as these typically require considerably higher tolerance.
One solution to the issue described above is the use of grating coupler embedded platforms. The grating coupler is typically realized by forming a grating structure on top of, or below, a waveguide. The incident light hits the grating at an angle of incidence with respect to the grating. For specific combinations of incident angles and light frequency, there is resonance, allowing the grating to couple light into a guided mode of the waveguide.
For design considerations of the essentially two-dimensional photonic integrated circuits, the grating coupler introduces a challenge as the direction of the incident light is different from the plane of the waveguide (i.e. the plane of the circuit board). To solve this problem, an optical arrangement can be added to deflect the light beam from a direction parallel with the plane of the waveguide (for example at an output of a semiconductor laser), into a direction forming an angle with respect to the waveguide for input to the grating coupler. The grating coupler then alters the direction of the light beam back into a direction in the plane of the waveguide.
In one proposed approach, an incident light beam is initially in the plane of the waveguide. A mirror forming an angle below 45° with respect to the plane of the waveguide is used on top of a grating coupler to deflect the incident light beam such that it hits the grating coupler at an angle of incidence which gives an efficient light coupling. The mirror is part of an additional wave guide attached on top of the grating coupler, making the solution relatively complex. Moreover, the angle below 45° required for efficient light coupling is difficult and relatively expensive to manufacture. For example, the mirror has to be formed by a dicing technique with a fine grid size blaze or patterned with a gray scale mask. There is thus room for improvements.